1. Field of the Invention
The object of the present invention is a display screen without moire effect. It finds an application in the production of any display devices, notably with micropoints, also referred to as the field emission display type (or FED for short).
"Without" moire effect means a moire effect which is sufficiently attenuated so as not to be visible to an observer.
2. Discussion of the Background
Although the invention is not limited to this type of display, it is in the case of field emission display screens that the state of the art will be described.
A field emission display screen is described notably in the document FR 2 623 013. The essentials of this device are depicted in the accompanying FIGS. 1 and 2.
The device depicted in these figures comprises, on a substrate 2, for example made of glass, a thin layer of silica 4 and, on this layer, a plurality of electrodes 5 in the form of parallel conductive bands fulfilling the role of cathodic conductors and constituting addressing columns.
These cathodic conductors are covered with a continuous resistive layer 7 (except on the ends to allow the connection of the cathodic conductors with biasing means 20). An electrically insulating layer 8, made of silica, covers the resistive layer 7.
Above the insulating layer 8 there are formed a plurality of electrodes 10 also in the form of parallel conductive bands. These electrodes 10 are perpendicular to the electrodes 5 and fulfill the role of a grid constituting the addressing lines.
The device also has a plurality of elementary emitters of electrons (micropoints), only one example of which (for reasons of simplification) is depicted schematically in FIG. 2: in each of the intersection areas (corresponding to an image point or pixel) cathodic conductors 5 and grids 10, the resistive layer 7 corresponding to this area supports micropoints 12, for example made of molybdenum, and the grid 10 corresponding to the said area has an opening 14 opposite each of the micropoints 12. Each of the latter adopts substantially the shape of a cone whose base rests generally on the layer 7 and whose apex is situated level with the corresponding opening 14. Naturally, the insulating layer 8 is also provided with openings 15 allowing passage of the micropoints 12.
This first subassembly defined by the area of intersection of the cathodic conductors and grid conductors 10, possibly associated with other elements, for example a supplementary grid within the screen or a filter on the face of the screen observed, can be referred to as an "intermediate subassembly".
Thus each intermediate subassembly corresponds to a pixel. Opposite this intermediate subassembly, there is a substrate 30 covered with a conductive layer 32 serving as an anode. This layer is covered with a layer or bands of luminescent materials 34. Hereinafter the emissive part opposite the pixel (or intermediate subassembly) will be referred to as the "anode subassembly".
In the case of a monochrome screen, or an unswitched three-color screen, the size of the anode subassembly corresponds to that of the intermediate subassembly. In the case of a switched three-color screen, the pixel is opposite three bands of luminescent materials, only one of which emits at a time, and the anode subassembly corresponds to the excited band part.
The light emitted by the luminous materials under the impact of the electrons emitted by the micropoints is received by the observer 0. In the usual case, the observation takes place on the anode side, and therefore through the anode subassembly, on the side opposite the excitation of the luminescent materials. However, the major part of the light being emitted on the excitation side, the result is that it is highly advantageous to observe this screen on the excitation side of the luminescent materials, and therefore through the intermediate subassembly which, because of this, must be at least partially transparent. This operating mode is all the more advantageous since the entire quantity of light emitted can be reflected towards the intermediate subassembly by the use of a reflective layer disposed behind the luminescent materials (this layer can be the anode itself or a supplementary layer, for example of aluminium). In addition, as the intermediate subassembly is partially transparent, it fulfills the role of a neutral filter and thus reduces the effects related to diffuse reflection, in the case notably where the luminescent materials are powder luminophores.
The intermediate subassembly defined by the intersection of an addressing row and column can take various forms. In one embodiment described in the document FR-A-2 687 839, the cathodic conductors have a lattice structure and the grid conductors a perforated structure. This embodiment is illustrated in FIGS. 3A and 3B, which are respectively plan and cross-sectional views.
In these figures, the cathodic conductors bear the reference 5a and the grid conductors the reference 10g. The grids have openings 11 opposite the areas of intersection of the conductive tracks 5a and are centered on these areas, as can be seen in FIG. 3A. Naturally, the grids also have holes 14a respectively opposite the micropoints 12.
More precisely, each grid 10g has substantially the structure of a lattice identical to the lattice of the corresponding cathodic conductor, but the lattice of this grid is offset, with respect to the lattice of the cathodic conductor, by a half-pitch parallel to the addressing rows and a half-pitch parallel to the addressing columns. Above an area where micropoints are collected, this grid has, in plan view, a square surface 10a which has holes 14a in it and at which there end four tracks 10b forming part of the lattice of this grid.
Many other embodiments are possible, but it will be understood that the intermediate subassembly, through which the observation is effected, though it is semi-transparent overall, is, in reality, formed by highly diverse areas if it is examined on a small scale. Each pixel defined by the overlapping of an addressing row and column therefore comprises a central area (which will be referred to as the "pupil" of the pixel) and four lateral half-parts. The four lateral parts separate each area from its four neighbours. Each pixel therefore has an optical transmission which is not uniform. FIG. 4 shows the appearance of such a pixel, where the central part 40 can be discerned, with its repetitive subassemblies corresponding to the meshing of the grid conductor and of the cathode conductor, and the lateral parts 42a, 42b, 42c, 42d.
This complex periodic structure of the intermediate subassembly, superimposed on the structure, also periodic, of the anode subassembly (in terms of emission as previously defined) can lead to display defects due to moire effects. These defects are illustrated in FIGS. 5a and 5b, on the one hand, and in FIG. 6 on the other hand.
FIGS. 5A and 5B, first of all, correspond to the case where the intermediate subassembly and the anode subassembly are not strictly aligned. This appears when there are several bands of luminescent materials (three-color screen, switched or otherwise). It is assumed that the columns of luminescent materials disposed on the anode are not strictly parallel to the addressing columns of the intermediate subassembly. FIGS. 5A and 5B are sections along a plane perpendicular to the columns, at two different points on the screen (for example at the top and at the bottom).
In these two figures, the intermediate subassembly bears the general reference 50 and is depicted schematically with regions 52 corresponding to the central part of the pixels, relatively transparent areas, and half-regions 54 corresponding to the lateral half-areas, less transparent; the anode subassembly is depicted in the form of the emissive luminous band 62.
FIGS. 5A and 5B depict, by way of example, the case of a three-color screen, switched or otherwise.
Since the light is not transmitted from the luminescent bands to the observer in the same way from one end of the column to the other, the image perceived will be interfered with by lines or fringes which are more or less bright and coloured. This moire effect is a nuisance to the observer.
FIG. 6 also shows, in the case of both a monochrome screen and a three-color screen, switched or otherwise, that the light flux emitted by an anode subassembly 62 is not strictly the same in the direction of an observer 70 facing the subassembly 62 and in the direction of an observer 72 placed to the side, whatever the direction of the movement.
The variations in transparency on the pixel scale and on the scale of each anode subassembly therefore gives rise to parasitic effects, which impair the quality of the displayed image.
The aim of the present invention is precisely to remedy these drawbacks.